Isolated Ground in Metal Conduit

[wilbur101]

I have a design for an isolated ground system. The building has a ground loop with ground rods. An additional ground rod is connected to the ground loop and then brought into the building (insulated wire) and connected to a panel. The panel is grounded to building steel, but the terminals for the isolated ground are isolated from the panel. The isolated ground is for I/O in a Delta V control panel. The Delta V panel is fed by a 120V ups ckt (hot, neutral + ground). The ground for the shields for the I/O cables are taken to the isolated ground panel mentioned above. Is there a problem running the isolated (insulated wire) ground conductor in metal conduit (insulated wire cannot be bonded to conduit due to design requirement)? Also, the conduit carrying the isolated ground wire is connected to the 120V UPS power conduit. The isolated ground is in conduit by itself for about 30 feet and then connects to the conduit system for the 120V UPS power and runs for about 70 feet to get to the delta v panel.

 

[bwyllie]

Re: Isolated Ground in Metal Conduit

I believe for a proper IG instalaltion the IG wire needs to be run with the circuit conductors/feeders.

 

[tom baker]

Re: Isolated Ground in Metal Conduit

An equipment grounding conductor, isolated or not, has to run with the circuit conductors inside the raceway, or the impedance will be so high the fault woun't clear. There is an exception to allow the EGC on the exterior, but its limited to 6 ft. See 250.102(E) and 250.96(B), and 250.134 (B)
Also the isolated system must be bonded to the electrical system grounded neutral, service enclosure, grounding electrode conductor, otherwise a fault will not clear, the NEC specifially states the following:
250.6 (D)Limitations to Permissible Alterations. The provisions of this section shall not be considered as permitting electronic equipment from being operated on ac systems or branch circuits that are not grounded as required by this article. Currents that introduce noise or data errors in electronic equipment shall not be considered the objectionable currents addressed in this section.

I if understand your design, the Delta V grounding system is isolated from the electrical grounding system, this is a violation of the NEC for several reasons.

[ November 05, 2003, 03:13 PM: Message edited by: tom baker ]

 

[steve66]

Re: Isolated Ground in Metal Conduit

I think the section on grounding is by far the most confusing part of the NEC, but here goes:

Tom, wouldn't the UPS be a separately derived sysetm? Wouldn't that make the ground wire a Grounding Electrode Conductor? Would this allow it to be ran separate from the feeder?

You mentioned:

... the Delta V grounding system is isolated from the electrical grounding system ...

But the way I understand the question, the ground wire (IMO we should call it a grounding electrode conductor) is ran back to the building Grounding Electrode system. So it isn't really isolated.

Steve

 

[ron]

Re: Isolated Ground in Metal Conduit

but the terminals for the isolated ground are isolated from the panel

At some point, this must be connected to the grounding system, generally where the n-g bond is in the UPS.

 

[dereckbc]

Re: Isolated Ground in Metal Conduit

Wilbur, it looks like you have some of use confused including myself. Sounds to me like you have a logic/shield ground which is not associated with any phase circuit conductors. If that is the case, the previous post by members are in error stating it has to be run with circuit conductors. NEC does not regulate this type of ground sub-system other than to say it shall be bonded to the common electrode system.

It would help if you could break the various ground sub-systems down. For example the UPS may need a GEC from the ground electrode sub-system (what you are calling a loop). I would not run any conductor from the ground electrode sub-system in conduit unless bonded at both ends.

Need some more info to help you out. Questions:
1. Are you supplying IGR circuits?
2. Is your UPS configured as SDS?
3. Is the isolated ground you speak of used as a shield/logic ground, and/or IGR?
4. How is the AC service grounded circuit conductor (neutral) bonded to the ground electrode system.

Tom made a very good point you cannot have truly isolated ground systems. They all have to be bonded together, otherwise you will put personnel and equipment at great risk.

 

[wilbur101]

Re: Isolated Ground in Metal Conduit

I'm glad somebody else is confused, I'm sick of being the only one!

1. Are you supplying IGR circuits? I don't think so - IGR Isolated Ground Receptacles?

2. Is your UPS configured as SDS? UPS is 480 in/480 out and feeds a 480-208Y/120 xfmr which is a separately derived system, solidly grounded to building steel which is connected to the building ground loop.

3. Is the isolated ground you speak of used as a shield/logic ground, and/or IGR? Shield/logic ground.

4. How is the AC service grounded circuit conductor (neutral) bonded to the ground electrode system? The neutral is developed at the xfmr and connected to building steel.

The isolated ground wire is connected to the building ground loop, which is the reference point for the whole building and all the systems inside the building.

 

[dereckbc]

Re: Isolated Ground in Metal Conduit

OK we are getting some where now. Lets go back to the isolated ground bar in the panel.

What kind of panel?
The GEC from the ground ring, is it connected to the isolated ground bar?

I am not clear how the ground bar is terminated as to whether it is connected to the GEC from the ground ring, building steel, or floating.

 

[wilbur101]

Re: Isolated Ground in Metal Conduit

The ground system starts with the loop outside the building. A bare copper 4/0 is buried ~ 3' below grade. 3/4" x 10' long copper ground rods are "pounded" into the earth and connected to the ground loop at specified intervals. An insulated 4/0 is connected to one of these ground rods. This insulated 4/0 is brought into the building to be used as the isolated ground for the shield ground of the delta v controller cabinets. The insulated 4/0 is brought to a hoffman box. The box is grounded to building steel. The box has Ilsco terminals that allow multiple conductors to be tapped off the 4/0. The Ilsco terminals are isolated from the backpanel of the hoffman box. A #8 insulated conductor is then routed to the delta v cabinet. The delta v cabinet is a controller cabinet with I/O cards. The panel has an equipment ground brought in separate. The insulated #8 is connected to a manufacturer provided ground bar that is isolated from the cabinet. The only connections to this isolated bar are the logic/shield grounds.

 

[dereckbc]

Re: Isolated Ground in Metal Conduit

Wilbur, I got the picture now, thanks. The ground cable you are referring too is not a GEC by NEC definitions IMO, therefore there is no requirement to bond it to the conduit.

Technically being from the telecom sector as a CO power engineer, I have questions as to why the manufacture would not want it bonded to the conduit or have it ran in PVC, due to choke effects. And why they did not originate the logic/shield ground from the same point where the grounded circuit conductor is bonded to form a isolated SPG. Signal, logic, shield, and equipment grounds are in reference to the power source, rather than earth. The way it is configured might actually create noise problems, but I do not see any safety problems provided the AC service ground is bonded to the ground ring.

Good Luck

Dereck

 

[wilbur101]

Re: Isolated Ground in Metal Conduit

Thanks

 

[gwz2]

Re: Isolated Ground in Metal Conduit

If the Ground conductor is insulated and in a metal raceway, I believe the raceway must be connected to the ground conductor other-wise what keeps the metal raceway at the ground potential ( equal-potential ).

If this were a GEC, then the GEC would need to be connected to each end of the metal enclosure ( 250.64(E) ). But since it is an ground conductor ( not even an EGC which is required to be installed "with the circuit conductors" 250.134(B) ), the metal enclosure must be grounded, at least on one end, to the ground conductor. 250.4(A)(4).

gwz2

 

[pierre]

Re: Isolated Ground in Metal Conduit

Could someone here explain what a Delta V ground system is? I have now heard this term used twice in the last month, and that is the only time I can recall hearing of it.
Thanks

Pierre

 

[dereckbc]

Re: Isolated Ground in Metal Conduit

gwz2, If you go back and read the discriptions the conduit he is using is grounded via the grounded panels. At least that is the way I understood the situation.